A lot of the time it's just what the gun looks like size wise, example is the LCP, the old model just looks a lot smaller tha the new one. I have the Champion, "which makes a great back up gun", even though I got the 2nd one for my wife, after selling the first one when they first came out, and it was a let down from my seacamp. Now I don't know if the new model is larger in real life, but it looks larger, "more formal" than the Champion I have. It's perception sometimes, not reality. Some guns just look a lot larger than they are.My Glock 30 S, is a small gun for a 10+1, 45 caliber gun. Now I haven't put it next to the M&P shield 45, but i would guess the Shield is a bit smaller, "but the Glock is only 20.8 ozs. , the shield is 22.5, I just peeked. So the Glock carries 10 and 1, the shield 6 plus one, or 7 plus 1. So the Glock 30S, is a lighter gun that carries 3 or 4 more rounds, which even I didn't expect until I looked on ,
http://concealednation.org/2016/07/firearm-review-smith-wesson-m-p-45-shield/, Looking at the guns in Magazines and print ads, the glock appears heavier and bigger, but when I lay my 30"S", next to my 26, it isn't much larger, it is closeer to a cub compact, than a compact, again perception vs reality.
I was going to buy a 45 shield until I read the specs, thinking it was more along the size of the old shield 40.So the whole thing is confusing unless you can lay them all next to each other, because guessing will always be a suprise, usually in a dissapointing way.
When I went to a glock 30 and then a 30S, it was because it was half the weight and held 3 more rounds than my 1911's, Which is the only good reason to me that I would opt not to carry my steel 38 oz gun. At gunfighting range it really won't matter how tight my groups are at 50 or 75 feet,